This invention relates to master cylinders for use in hydraulic systems such as hydraulic braking systems or hydraulic clutch actuating systems in automobiles or the like.
In a hydraulic system utilizing hydraulic fluid such as oil or the like as actuating fluid it is necessary to remove or expel air completely from the system including various components and pipe lines to operate the system satisfactorily. Usually, a bleed valve is provided in one or more components of the system for expelling air which has remained in the system whle filling hydraulic fluid into the system, but the air-bleeding operation is time-consuming and troublesome.
A method has been proposed for filling hydraulic fluid quickly and completely into a hydraulic system or into a hydraulic component such as a master cylinder or the like, in which, firstly, the system or the component is connected to a source of a high vacuum pressure (for example, 750 mmHg gauge or more) so as to substantially extract or remove air from the interior of the system or the component, and secondly, hydraulic fluid is supplied into the system or the component by, for example, exchanging a three way valve which is connected to the source of vacuum pressure, a hydraulic fluid supplying tank, and a suitable fitting formed or connected to the system or the component. The method is particularly adapted for filling hydraulic fluid into a newly assembled hydraulic system or into a newly manufactured hydraulic component quickly and easily.
While, a master cylinder constituting one of the essential components in a hydraulic braking system or clutch actuating system has been formed such that a cup-shaped seal (usually called as a secondary cup) fitted on a piston acts to seal an oil chamber defined in a cylinder in which the piston works from the outside of the master cylinder, the secondary cup is disposed essentially to prevent leakage of oil from the oil chamber to the outside. Thus, when vacuum pressure is applied in the oil chamber according to the aforementioned method, atmospheric air will pass through the secondary cup relatively easily, whereby it is not possible to attain a desired high vacuum in the system or in the oil chamber of the master cylinder, thus causing the presence of residual air when oil is filled into the system or into the master cylinder thereafter.
An object of the present invention is to provide an improved master cylinder elminating the aforementioned shortcomings.